1. Filed of the Invention
The present invention relates to a control circuit of forward power converter, and more particularly, to a synchronous rectifier control circuit for a forward power converter for improving efficiency of power conversion.
2. Description of the Related Art
Power converters have been frequently used for converting an unregulated power source to a constant voltage source and/or a constant current source. A transformer having a primary winding and a secondary winding is typically used for power conversion. In a typical application, the primary winding is coupled to an unregulated power source, preferably a DC voltage source, and a switching device is connected to the primary winding for switching on and off the conduction between the power source and the primary winding. A rectifying diode is typically connected to the secondary winding for rectifying the energy converted from the primary winding into a DC voltage. However, a forward voltage drop across the rectifying diode inevitably causes a conduction loss and renders the rectifying diode as the key component for producing the loss. To resolve the problem of power loss, low-on-resistance transistor has been used to replace the rectifying diode and to provide a synchronous rectification of power converter. Recently, a synchronous rectifying technique is proposed in “Single ended forward converter with synchronous rectification and delay circuit in phase-locked loop” by Edgar Abdoulin, U.S. Pat. No. 6,026,005. However, the drawback of the aforementioned conventional synchronous rectifying technique is the reduced power efficiency under light load conditions. Furthermore, a cross conduction may occur during heavy load operations.
FIG. 1 illustrates a conventional forward power converter having a synchronous rectifier (SR). The forward power converter comprises a transformer 10, a plurality of switching devices 20, 30 for controlling the conduction between the primary winding of the transformer 10, and an input voltage source VIN. A plurality of diodes 25 and 35 are applied for retrieving the inductance energy of the primary winding back to the input voltage source VIN. Two transistors 60, 70, operating as a synchronous rectifier, are connected to the secondary winding of the transformer 10. The first transistor 60 is coupled between a first terminal of the secondary winding and a ground terminal. The second transistor 70 is connected from a second terminal of the secondary winding to the ground terminal. An inductor 80 is coupled between the second terminal of the secondary winding and an output terminal of the power converter. An output capacitor 85 is disposed between the output terminal of the power converter and the ground terminal. FIG. 1A illustrates the first operational stage of the conventional power converter. In this stage the switching devices 20 and 30 are turned on to conduct energy from the input voltage source VIN to the output terminal of the power converter through the transformer 10 and the inductor 80. The transistor 60 is turned on to operate as a synchronous rectifier after its parasitic diode 65 is conducting. FIG. 1B illustrates the second operational stage of the conventional power converter. In this stage the switching devices 20, 30 are turned off. The energy stored in the inductor 80 is continuously discharged to the output terminal of the power converter through a parasitic diode 75 of the transistor 70. The transistor 70 is turned on to operate as a synchronous rectifier after its parasitic diode 75 is conducting.
The forward power converter normally has two different operation modes, namely a discontinuous operation mode and a continuous operation mode. In the continuous operation mode, the energy remains in the inductor 80, that is, the next cycle begins before the current released from the inductor 80 reaches zero. Because the transistor 75 is switched on to operate as a synchronous rectifier during the second operation stage, therefore, a cross conduction may occur after the start of the next cycle as illustrated in FIG. 2A, in which the secondary winding is short-circuited through the transistor 70 and the parasitic diode 65. During cross conduction, an EMI (electromagnetic interference) shall be generated and the lifespan of the transistors 70, 60 shall be severely reduced. In contrast, while in the discontinuous operation mode, all of the energy stored in the inductor 80 is completely discharged before the next cycle starts. Therefore, no inducted voltage remains in the inductor 80 to resist the energy of the output capacitor 85 discharging back to the transformer 10. As illustrated in FIG. 2B, when the power converter is in discontinuous operation mode under light load conditions, the energy of the inductor 80 is completely delivered at the switching instance and a reverse current shall be discharged from the output capacitor 85 to the transformer 10. The reverse current produces power losses and dramatically reduces the efficiency of power conversion. The objective of present invention is to provide a control circuit for synchronous rectifying, which has higher operation efficiency at both the continuous operation mode and the discontinuous operation mode.